1. Technical Field
The present invention relates to a line type thermal print for printing in a facsimile or the like, the line type thermal print head comprising a heating resistor and circuit patterns corresponding to respective portions of the heating resistor.
Also, the present invention relates to line type thermal print head device, including the above mentioned line type thermal print head and a head connector which is mounted on the print head in such a manner as to conduct to each of the connection terminals of the line type thermal print head.
2. Background Art
(1) Line Type Thermal Print Head
Generally, a thermal print head of this type has a rectangular head circuit board and a heating resistor formed in a straight line in the longitudinal direction (line-direction) of the circuit board. A common wiring pattern is formed so as to extend in parallel with the heating resistor along one side edge in the longitudinal direction of the surface of the head circuit board, and the common wiring conducts to those portions of the heating resistors which correspond to the respective print dots. Further, a plurality of individual circuits are formed in the area opposite to the common wiring across the heating resistor on the surface of the head circuit board. The individual circuits conduct to those portions of the heating resistor which correspond to the respective dots. Further, a ground wiring pattern is formed so as to extend in the longitudinal direction of the surface of the head circuit board, and a plurality of driver elements are mounted on the head circuit board which connect the ground wiring to the individual circuits.
When each driver element switches from one circuit to another under the condition that the line type thermal print head is mounted in the printer of a facsimile or the like, an arbitrary individual circuit electrically conducts to the ground wiring. Accordingly, the portion of the heating resistor which corresponds to the currently conducting individual circuit generates heat, so that printing takes place at the print dot according to the amount of heat generated.
The ground wiring and the common wiring in the line type thermal print head extend along the linear heating resistor. Both wiring patterns are connected to a power source so as to have electrical power supplied at one end or both ends thereof. Since both wiring patterns are thin and long and narrow, it is difficult to reduce the electrical resistance of both wiring patterns. Therefore, under this arrangement, there is a voltage drop along the longitudinal direction of each wiring pattern. Owing to the voltage drop, a difference in density occurs in the printed dots.
Japanese Patent Laid-Open Publication No. Hei 2-286261 (Japanese Patent Publication No. Hei 4-17797) proposes a line type thermal print head developed to solve the above-mentioned problem (Related Art 1). FIG. 12 shows the outline of this line type thermal print head. In FIG. 12, reference numeral 201 denotes a head circuit board, 202 a heating resistor, 203 a common wiring pattern, 205 driver elements, and 207 a ground wiring pattern, with no individual circuits depicted. As shown in FIG. 12, the ground wiring pattern 207 is divided substantially at the center of the head circuit board 201 into a right-side ground wiring pattern 207a and a left-side ground wiring pattern 207b. A pair of ground connection terminals 208 and a pair of common-wiring connection terminals 212 are formed close to the longitudinal side edge 201b and substantially in the central area in the longitudinal direction of the head circuit board 201. The pair of common-wiring connection terminals 212 are located more towards the inside than the pair of ground connection terminals 208. The pair of ground connection terminals conduct to the pair of ground wiring patterns, respectively, while the common-wiring connection terminals conduct to both ends of the common wiring pattern 203.
In Related Art 1, the voltage drop in the common wiring pattern 203 becomes larger as one moves from either side toward the center area, and the voltage drop in the pair of ground wiring patterns becomes larger as one moves toward either side. Therefore, the voltage between the common wiring pattern 203 and each of the pair of the ground wiring patterns 207 is maintained substantially at a fixed level along the longitudinal direction of the head circuit board 201, so that the difference in printing density resulting from the voltage drop is small.
However, with the arrangement of Related Art 1 that the ground wiring pattern 207 is divided into a right-side pattern 207a and a left-side pattern 207b, there is a problem. In a case where printing is carried out using this thermal line type thermal print head, it sometimes happens that the number of dots in printing with the right-side ground wiring pattern 207a differs from the number of dots in printing with the left-side ground wiring pattern 207b. In this case, a voltage difference occurs between the ground wiring pattern in which more dots are used and the ground wiring pattern in which less dots are used. For example, this voltage difference can be expressed as a difference between representative voltage values of both ground wiring patterns, the representative value being a voltage at the center of each ground wiring pattern. This voltage difference at times causes a density difference between printing by one ground wiring pattern and printing by the other ground wiring pattern.
Another problem with Related Art 1 is as follows. In Related Art 1, both common-wiring connection terminals 212 are located more towards in the inner positions than the ground connection terminals 208. Between each ground connection terminal 208 and the side edge 201b of the head circuit board 201, there is the common-wiring connection circuit 211 to connect both ends of the common-wiring pattern 203 to both common-wiring connection terminals 212. Because of this structural restriction, it is impossible to arrange the circuits so that both ground connection terminals 208 extend up to the side edge 201b of the head circuit board 201. In order to connect external wires (or the connector for external wires) to both ground connection terminals 208, it is necessary to arrange such a connection circuitry to go beyond the common-wiring connection terminals 211. It is required to do away with such a complicated connection, and facilitate connection of various kinds of external wires to the ground connection terminals 208.
(2) Line Type Thermal Print Head Assembly
The line type thermal print head assembly is a system having a head connector mounted to the line type thermal print head. The head connector is mounted so as to conduct to the connection terminals of the line type thermal print head. External wires (or the terminal connector of external wires) are connected to the head connector.
In most cases, the head connector is conventionally mounted to the side edge of the line type thermal print head. The head connector has the connector body made of a hard insulating material, such as a hard synthetic resin, and includes a plurality of terminal metal parts protruding from the connector body. On the other hand, in the line type thermal print head, there are provided through-holes at positions corresponding to the terminal metal parts. The head connector is fixed to the line type thermal print head after its terminal metal parts are inserted into the through-holes of the print head by soldering. In this arrangement, it is necessary to provide the through-holes in the line type thermal print head, and insert the protruding ends of the terminal metal parts into the through-holes. This has been troublesome and a considerably wide space has been required for mounting the head connector.
In a system proposed in Japanese Patent Laid-Open Publication Nos. Hei 6-246948 and Hei 6-267620 (Related Art 2), a terminal metal part protrudes from a connector body made of an insulating material, such as a hard synthetic resin as shown in FIG. 13. In FIG. 13, reference numeral 301 denotes a head circuit board, 303 a head connector, 305 a connector body, and 307 a terminal metal part. The terminal metal part is mounted so as to protrude in a direction toward the surface of the circuit board of the print head (in parallel with the surface of the circuit board). The protruding part of the terminal metal part is shaped in a bifurcated form. The bifurcated portions of the terminal metal parts are fitted over the side edge of the print head. As the terminal metal parts clasp the side edge portion of the print head, the head connector is mounted. In contrast to the conventional system, in this system the head connector can be mounted easily and a smaller space is required for mounting the connector.
However, in spite of the advantage mentioned above, the system of Related Art 2 has the following problem. In the above system, the bifurcated portions of the terminal metal parts protrude from one side of the connector body, the head connector is mounted to the line type thermal print head using only the bifurcated portions. Therefore, it is not easy to mount the head connector in a stable condition, to secure sufficient mounting strength or prevent the head connector from becoming shaky.
Further, in the above system, the direction of fitting the terminal metal parts of the head connector on the line type thermal print head is in the direction of the flat surface of the circuit board. In other words, this fitting direction is the direction in which the terminal metal parts protrude from the connector body. On the other hand, the direction of inserting and removing the external wires (or the terminal connector of external wires) to and from the head connector is generally the same as the protruding direction of the terminal metal parts. Therefore, when inserting or removing the external wires to or from the head connector, it is necessary to take care that the head connector does not come off the print head. There has been demand for a line type thermal print head system without a possibility of the connector coming off.